1. Technical Field
The present invention relates to a method for producing a particulate water-absorbing agent composed principally of a water-absorbing resin.
2. Background Art
In recent years, in hygiene products such as disposable diapers, sanitary napkins and incontinent pads, the water-absorbing resin as a composition material thereof has been widely used, as a water-absorbent agent, in view of absorbing a body liquid. As such a water-absorbing resin, for example, there have been known a cross-linked product of partially neutralized polyacrylic acid, a hydrolysate of a starch-acrylic acid graft polymer, a saponified product of a vinyl acetate-acrylate ester copolymer, a hydrolysate of an acrylonitrile copolymer or an acrylamide copolymer and a cross-linked product thereof, and a cross-linked product of a cationic monomer and the like. This water-absorbing resin may be used after converted to also a sheet-like, fiber-like or film-like form, however, it is generally used in a water-absorbing agent after converted to a powder-like form (particulate). As such powders (particles), for example, the particulate water-absorbing agent, having a weight average particle diameter thereof of 200 to 800 μm, is widely used. The particulate water-absorbing agent is produced via many steps. These steps may include the polymerization step, the drying step, the pulverization step, the classification step, the surface cross-linking step, and the like.
The particulate water-absorbing agent has been produced under control of various parameter properties (for example, absorbency, absorbency against pressure, water absorbing speed, liquid permeation, gel stability and the like), as specifications, depending on intended use (for example, disposable diapers, sanitary napkins and the like). However, in a large scale continuous production of the particulate water-absorbing agent, enhancement of productivity thereof, enhancement of property and stabilization of property are difficult, and a small deflection of property may incur property decrease of a final product (for example, disposable diapers and the like) or consumer claims. Maintaining of high property and stable continuous operation have been big problems. In order to solve these problems, Patent Document 2 has disclosed a method for removing the water-absorbing resin having property out of the range of the upper and lower limits, and mixing them again. Patent Document 3 and 4 has disclosed technology of using a plurality of hoppers in an intermediate step or a polymerization step. In addition, Patent Document 5 has disclosed a method for performing polymerization in two lines, and the latter half in one line. Furthermore, many proposals have been provided on technology for enhancement and stabilization of property of the particulate water-absorbing agent, by changing or furnishing a new intermediate production step, in Patent Documents 2 to 4 or the like, as well as Patent Document 1, or by using in combination of a plurality of production apparatuses in Patent Documents 4, 6 to 8 or the like, however, there was sufficient room left to be improved.
In addition, technology of removing and recycling fine powders again, after surface cross-linking for property enhancement, has been proposed in Patent Document 9 or the like, however, removal of the fine powders has a problem of decrease in yield.